The present invention relates to a distance measuring apparatus for measuring a distance to an obstacle such as an automobile or motorcycle, a person, a structure or building beside a road or the like, on the basis of the reflected light from the obstacle, caused by irradiation of a light beam from the apparatus.
Such a distance measuring apparatus is used for a "following" distance sensor or an auto-cruising system. Generally, the distance measuring apparatus has a light transmission optical system for transmitting a fixed or scanned light beam and a reception system for receiving the light reflected from the obstacle, so as to measure the distance to the obstacle by counting the time in which the light moves between the apparatus and the obstacle. The light beam transmitted from the light transmission optical system generally has a Gaussian light intensity distribution or the like, in which the intensity is largest at the center, and becomes reduced as the distance from the center is increased.
When the obstacle crosses the light beam having the Gaussian light intensity distribution or the like, stable measurement is executed after transitionally unstable measurement has been executed. As the obstacle overlaps a part of the light beam, dispersion in the measurement values is large since the quantity of the scattered light is insufficient. As the rate of the obstacle crossing the light beam becomes larger, the quantity of the scattered light becomes increased and stable measurement can be executed with little dispersion.
When the monitoring area is a middle distance or a long distance, there is enough time in collision of mobiles moving at a high speed, and measurement can be executed sufficiently by software processing such as statistical processing of the measurement data.
However, if the movement of the obstacle including another moving body is kept until the moment immediately before a collision to avoid the collision or drive a life saving device, the monitoring area needs to be also a short distance, and the width of an own vehicle or a running lane needs to be also monitored.
The short distance indicates, herein, a distance of up to about 30 m. From a distance of 30 m, for example, automobiles running at 50 km/h collide in about one second, and automobiles running at 100 km/h collide in 0.5 second. The movement of a moving body in 10 m particularly has great influence in terms of avoidance of the collision or drive of a life saving device.
In such a case, it is preferable that the transitionally unstable measurement time mentioned above should be as short as possible.
Incidentally, an area for monitoring the obstacle is demanded to be as large as possible. On the other hand, a small monitoring area has an advantage that an algorithm for judging the danger of the collision may be simplified. In consideration of these factors, the monitoring area is, for a short distance as mentioned above, preferably the width of an own automobile.
Generally, the degree of danger of life is high in the collision against an obstacle in a 1 m range with respect to the center of the width of an automobile, and low in the collision against an obstacle out of the range, so called, in an offset state.
When the monitoring area is set as described above, the transitionally unstable measurement time mentioned above is required to be as short as possible. In addition, it is requested that this should be implemented at costs that are as low as possible.